1. Field of the Invention
Priority is claimed to Japanese application No. 2004-31309, filed Feb. 6, 2004, which is incorporated herein by reference.
The present invention relates to a DC/DC converter, and in particular relates to a voltage step-up DC/DC converter that is further miniaturized and lightened, and a program.
2. Description of Related Art
Conventionally various types of voltage step-up DC/DC converters have been proposed. Many of the conventional voltage step-up DC/DC converters are provided with a basic construction as shown in FIG. 7, as shown, for example, in Japanese Patent Application, first Publication No. 2003-111390 and Japanese Patent Application, First Publication No. 2003-216255.
The voltage step-up DC/DC converter shown in FIG. 7 (also referred to simply as a “DC/DC converter”) comprises; a capacitor Cin on an input side, a capacitor Cout on an output side, a coil (inductor) L, a switch SW1 with a transistor, and a diode D1.
In FIG. 7, when the switch SW1 is turned ON, electric current flows as: DC power supply (electric charge accumulated in the capacitor Cin)→coil L→switch SW1→GND. At this time, the coil L is subjected to direct current excitation, and magnetic energy is accumulated.
Next, when the switch SW1 is turned OFF, the induced voltage generated by the magnetic energy accumulated in the coil L is superimposed on the power supply voltage (the voltage of Cin), a voltage higher than the input voltage value from the power supply is output from the coil L, and an output electric current Ic is output via the diode D1.
Moreover, by changing an ON/OFF duty ratio of the switch SW1, a required output voltage can be obtained within a predetermined range. FIG. 8 is a timing chart showing time variance in output electric current Ic in the circuit of FIG. 7. It can be seen that a greater output electric current Ic is output during the OFF period of the switch SW1 than during the ON cycle.
This DC/DC converter circuit shown in FIG. 7 is a practicable voltage step-up circuit, and is conventionally well known.
However, since the above mentioned circuit shown in FIG. 7 is constructed such that the electric current from the power supply, simply direct-current-excites the coil L and energy is accumulated, the size of the coil L must be large in order to prevent magnetic saturation in the coil L when it is being direct-current-excited. Moreover, generally the price of the coil rises in accordance with its size. Hence, miniaturization and price reduction of an entire DC/DC converter circuit becomes difficult.
Therefore, a construction may be considered in which two coils (inductors) are provided, as with the circuit shown in FIG. 9, and electric current I1 from the first coil L1 and electric current I2 from the second coil L2 are alternately output to the output side by alternately turning ON and OFF switches SW1 and SW2, which are provided on the output side of each coil. FIG. 10 is a timing chart showing time variance in output electric current in a circuit of such a constriction.
In FIG. 9 and FIG. 10, electric current flows in the coil L1 when the switch SW1 is turned ON, and when the SW1 is turned OFF, the magnetic energy accumulated in the coil L1 flows to the output side as electric current I1 via the diode D1. Correspondingly, electric current flows in the coil L2 when the switch SW2 is turned ON, and when the SW2 is turned OFF, the magnetic energy accumulated in the coil L2 flows to the output side as electric current I2 via the diode D2. Therefore, the electric current that flows to the output side is the Ic denoted in FIG. 10.
According to this construction, the electric current from the power supply is dispersed since the two coils are provided, and the peak electric current in each coil is decreased to below that shown for the construction of FIG. 7. However the switching frequency remains the same, and light weight material having low magnetic loss cannot be used for the core material. That is, since the circuit shown in FIG. 9 is a type in which magnetic energy is accumulated in the coils (inductors), the inductors have to be formed with cores of heavy material in order to accumulate sufficient magnetic energy without having magnetic saturation. This has been an obstruction to the miniaturization, lightening, and price reduction of the entire device.
Meanwhile, a technique in which a gap is formed in the core to purposely generate magnetic leakage to avoid magnetic saturation has been known conventionally. However, in this method, a highly advanced technique is required for the gap formation process (such as cutting the core), and control of the magnetic leakage when operating the circuit also requires a highly advanced technique. Moreover, in forming a gap, problems in respect of strength, cost and labor, and a decrease in rigidity of the core and the like occur.